0xe01cfbdd…0372sent to0x81c094f7…aa23·#22,935,943·view on Etherscan
Amniotes co-opted the ancestral body of anamniotes for their "developmental sac"
The amniote develops within a "developmental sac", the chorionic cavity, that forms from their own tissue (and also have an inner sac, the amniotic cavity). The evolutionary leap from the "naked" development of the anamniote (protected within an egg from maternal tissue) to the "developmental sac" of the amniote has not been explained. In evolution, the basic mode of operation is co-option, to use something that already exists and use it for a new purpoes. Where could the amniote find a "developmental sac"? Within the body of the ancestral anamniote itself. The chorionic cavity is actually the coelom of the anamniote, and the amniotic cavity is the neural tube of the anamniote, and the amniote develops with a "nested developmental program" within interaction between the neural tube (amnion) and gastric tube of the anamniote, where the "central organizer" - notochord - of the anamniote would have been positioned.